The role of substantia gelatinosa as a gate control

A gate control exists by which peripheral afferents and descending pathways can modulate sensory transmission. Evidence is presented that the mechanism may exist in the substantia gelatinosa laminae II and III. This area receives all known types of peripheral afferent from skin, from viscera and from high-threshold muscle afferents. The chemistry of the region is unique. Peripheral afferent terminals contain substance P, somatostatin, and fluoride-resistant acid phosphatase. Cells in the region contain enkephalin and GABA. At least three descending systems from the brainstem terminate in the area. The anatomical substrate exists by which cells in laminae II and III can receive afferents and descending axons and intrude onto cells of laminae I, IV, and V. Stimuli limited to the axons of laminae II and III cells in the Lissauer tract produce dorsal root potential and change the excitability of mono- and polysynaptic reflexes. They also change the excitability and receptive fields of cells in laminae IV and V. Recording from single units in laminae II and III reveals cells with many unusual properties not seen in the large dorsal horn cells. These unusual properties include small receptive fields, very prolonged responses to single stimuli, prolonged habituation, and shifting receptive fields. The action of the gate control shows it to be subtle and far beyond a simple control of overall excitability. Excitations and inhibitions are independently controlled. Different types of convergent afferent may be turned on and off. There are signs of both short-and long-lasting actions. It seems that a good case has been made for the cells of substantia gelatinosa taking part in the gate control mechanism.